1. Field of the Invention
The invention relates to relaxation oscillators and, more particularly, to relaxation oscillators which are synchronized to an external source.
2. Description of the Prior Art
Prior art designs of astable relaxation oscillators have employed external sources for controlling the operating frequency. This synchronization is readily accomplished with sinusoidal oscillators as the external source only over a narrow band of frequencies wherein the ratio of the nominal frequency of the relaxation oscillator to the frequency of the injected signal is a ratio of integers.
When the synchronizing source is not applied, the astable relaxation oscillator may continue to operate at a nominal frequency which is determined by the circuit values and the characteristics of the oscillating element. The period of this nominal frequency includes the interval during which the capacitor of the timing circuit charges to the firing voltage of the transistor employed as the oscillating device. Though this interval is essentially determined by the time constant of the timing circuit and the firing voltage of the transistor, it is modified by the current leakage at the transistor electrode to which the capacitor is connected. Since the leakage and firing voltage vary significantly between transistors of the same type, the replacement of the transistor generally alters the nominal frequency of the oscillator.